Sealable, biaxially oriented polyester film

ABSTRACT

The invention relates to a biaxially oriented, sealable polyester film with at least a base layer (B), with a sealable outer layer (A) and with another, nonsealable outer layer (C), where the sealable outer layer (A) has a minimum sealing temperature of not more than 110° C. and a seal seam strength of at least 1.3 N/15 mm of film width, and the topographies of the two outer layers (A) and (C) have particular characterizing features. The film of the invention is particularly suitable for use in flexible packaging and specifically in particular for use on high-speed packaging machinery.

[0001] The invention relates to a transparent, sealable, coextruded,biaxially oriented polyester film composed of at least a base layer (B)and, applied to the two sides of this base layer, outer layers (A) and(C). The invention also relates to the use of the film, and to a processfor its production.

BACKGROUND OF THE INVENTION

[0002] Sealable, biaxially oriented polyester films are known in theprior art. These known prior art films either have good sealingperformance or good optical properties or acceptable processingperformance.

[0003] GB-A 1 465 973 describes a coextruded, two-layer polyester film,one layer of which is composed of copolyesters containing isophthalicand terephthalic acids and the other layer of which is composed ofpolyethylene terephthalate. No useful information is given in thespecification concerning the sealing performance of the film. Due tolack of pigmentation, the film cannot be produced reliably (cannot bewound), and its capability for further processing is limited.

[0004] EP-A 0 035 835 describes a coextruded sealable polyester filmwhere, to improve the winding and processing characteristics, thesealing layer contains particles whose average particle size exceeds thethickness of the sealing layer. The particulate additives create surfaceprotrusions preventing the undesirable blocking and sticking of the filmto rolls or guides. No further information is provided on theincorporation of antiblocking agents with regard to the other,nonsealable layer of the film. It remains moot whether this layercontains antiblocking agents. Choosing particles having a largerdiameter than the sealing layer and the concentrations reported in theexamples has an adverse effect on the sealing characteristics of thefilm. The reference provides no information on the sealing temperaturerange of the film. The seal seam strength is measured at 140° C. andfound to be in the range from 63 to 120 N/m (which corresponds to 0.97to 1.8 N/15 mm of film width).

[0005] EP-A 0 432 886 describes a coextruded, multilayer polyester filmwhich has a first surface on which has been arranged a sealable layer,and has a second surface on which has been arranged an acrylate layer.The sealable outer layer here may also be composed ofisophthalic-acid-containing and terephthalic-acid-containingcopolyesters. The coating on the reverse side gives the film improvedprocessing performance. The patent gives no indication of the sealingrange of the film. The seal seam strength is measured at 140° C. For asealable layer thickness of 11 μm the seal seam strength given is 761.5N/m (11.4 N/15 mm). A disadvantage of the reverse-side acrylate coatingis that this side is now not sealable with respect to the sealable outerlayer, and the film therefore has only very restricted use.

[0006] EP-A 0 515 096 describes a coextruded, multilayer, sealablepolyester film which comprises a further additive on the sealable layer.The additive may comprise inorganic particles, for example, and ispreferably applied in an aqueous layer to the film during itsproduction. Using this method, the film is claimed to retain its goodsealing properties and to be easy to process. The reverse side comprisesonly very few particles, most of which pass into this layer via therecycled material. This patent again gives no indication of the sealingtemperature range of the film. The seal seam strength is measured at140° C. and is above 200 N/m (3 N/15 mm). For a sealable layer of 3 μmthickness the seal seam strength given is 275 N/m (4.125 N/15 mm).

[0007] WO 98/06575 describes a coextruded, multilayer polyester filmwhich comprises a sealable outer layer and a nonsealable base layer. Thebase layer here may have been built up from one or more layers, and oneof these layers is in contact with the sealable layer. The other(outward-facing) layer then forms the second nonsealable outer layer.Here, too, the sealable outer layer may be composed ofisophthalic-acid-containing and terephthalic-acid-containingcopolyesters, but these comprise no antiblocking particles. The filmalso comprises at least one UV absorber, which is added to the baselayer in a weight ratio of from 0.1 to 10%. The base layer of this filmhas conventional antiblocking agents. The film has good sealability, butdoes not have the desired processing performance and has shortcomings inoptical properties (gloss and haze).

[0008] It was therefore an object of the present invention to provide asealable, biaxially oriented polyester film which does not have thedisadvantages of the prior art films mentioned and which in particularhas improved sealability and improved processability, while otherwiseits optical properties remain the same or even improve. It was aparticular object of the present invention to extend the sealing rangeof the film toward low temperatures, and to improve the seal seamstrength of the film. In addition, the film was also to be processableon high-speed processing machinery. It should also be ensured that anycut material arising during film production can be reintroduced asrecycled material to the production process in amounts of up to 60% byweight, based on the total weight of the film, without adverselyaffecting the physical or optical properties of the film.

SUMMARY OF THE INVENTION

[0009] According to the invention, the object is achieved by providing acoextruded, biaxially oriented, sealable polyester film with at least abase layer (B), with a sealable outer layer (A) and with another outerlayer (C), where the sealable outer layer A has a minimum sealingtemperature of not more than 110° C. and a seal seam strength of atleast 1.3 N/15 mm of film width, and the topograhies of the two outerlayers (A) and (C) have the following features:

[0010] Sealable outer layer (A):

[0011] R_(a)≦40 nm

[0012] Value measured for gas flow within the range from 300 to 4000 s.

[0013] Nonsealable outer layer (C):

[0014] COF≦0.5

[0015] 40 nm≦R_(a)≦150 nm

[0016] Value measured for gas flow≦140 s.

[0017] Number of elevations N_(c) per mm² of film surface correlatedwith their respective heights h via the following equations:

A _(c1) −B _(c1)·log h/μm<log N _(c) /mm ² <A _(c2) −B _(c2)·log h/μm0.01 μm≦h≦10 μm

A_(c1)=0.29 B_(c1)=3.30

A_(c2)=1.84 B_(c2)=2.70.

[0018] The subclaims give preferred embodiments of the invention, andthese are described in more detail below.

DETAILED DESCRIPTION OF THE INVENTION

[0019] According to the invention, the film has at least three layersand then embraces the base layer (B), the sealable outer layer (A) andthe nonsealable outer layer (C).

[0020] Polymers used for the base and for the outer layer:

[0021] Base material:

[0022] At least 90% by weight of the base layer (B) is preferablycomposed of a thermoplastic polyester. Polyesters suitable for thispurpose are those made from ethylene glycol and terephthalic acid(polyethylene terephthalate, PET), from ethylene glycol andnaphthalene-2,6-dicarboxylic acid (polyethylene 2,6-naphthalate, PEN),from 1,4-bishydroxymethylcyclohexane and terephthalic acid(poly-1,4-cyclohexanedimethylene terephthalate, PCDT), or else made fromethylene glycol, naphthalene-2,6-dicarboxylic acid andbiphenyl-4,4′-dicarboxylic acid (polyethylene 2,6-naphthalatedibenzoate, PENBB). Particular preference is given to polyesters atleast 90 mol %, preferably at least 95 mol %, of which is composed ofethylene glycol units and terephthalic acid units, or of ethylene glycolunits and naphthalene-2,6-dicarboxylic acid units. The remaining monomerunits derive from other aliphatic, cycloaliphatic or aromatic diols and,respectively, dicarboxylic acids, as may also occur in the layer (A) orin the layer (C).

[0023] Other examples of suitable aliphatic diols are diethylene glycol,triethylene glycol, aliphatic glycols of the formula HO—(CH₂)_(n)—OH,where n is an integer from 3 to 6 (in particular 1,3-propanediol,1,4-butanediol, 1,5-pentanediol and 1,6-hexanediol) and branchedaliphatic glycols having up to 6 carbon atoms. Among the cycloaliphaticdiols, mention should be made of cyclohexanediols (in particular1,4-cyclohexanediol). Examples of other suitable aromatic diols have theformula HO—C₆H₄—X—C₆H₄—OH, where X is —CH₂—, —C(CH₃)₂—, —C(CF₃)₂—, —O—,—S— or —SO₂—. Bisphenols of the formula HO—C₆H₄—C₆H₄—OH are also verysuitable.

[0024] Other aromatic dicarboxylic acids are preferablybenzenedicarboxylic acids, naphthalene dicarboxylic acids (such asnaphthalene-1,4- or -1,6-dicarboxylic acid), biphenyl-x,x′-dicarboxylicacids (in particular biphenyl-4,4′-dicarboxylic acid),diphenylacetylene-x,x′-dicarboxylic acids (in particulardiphenylacetylene-4,4′-dicarboxylic acid) or stilbene-x,x′-dicarboxylicacids. Among the cycloaliphatic dicarboxylic acids mention should bemade of cyclohexanedicarboxylic acids (in particularcyclohexane-1,4-dicarboxylic acid). Among the aliphatic dicarboxylicacids, the C₃-C₁₉ alkanediacids are particularly suitable, and thealkane moiety here may be straight-chain or branched.

[0025] One way of preparing the polyesters is the transesterificationprocess. Here, the starting materials are dicarboxylic esters and diols,which are reacted using the customary transesterification catalysts,such as the salts of zinc, of calcium, of lithium, of magnesium or ofmanganese. The intermediates are then polycondensed in the presence ofwell-known polycondensation catalysts, such as antimony trioxide ortitanium salts. Another equally good preparation method is the directesterification process in the presence of polycondensation catalysts.This starts directly from the dicarboxylic acids and the diols.

[0026] Sealable outer layer (A):

[0027] The sealable outer layer (A) applied by coextrusion to the baselayer (B) is based on polyester copolymers and essentially consists ofcopolyesters composed predominantly of isophthalic acid units and ofterephthalic acid units and of ethylene glycol units. The remainingmonomer units are derived from other aliphatic, cycloaliphatic oraromatic diols and, respectively, dicarboxylic acids, as may also bepresent in the base layer. The preferred copolyesters which provide thedesired sealing properties are those composed of ethylene terephthalateunits and ethylene isophthalate units, and of ethylene glycol units. Theproportion of ethylene terephthalate is from 40 to 95 mol % and thecorresponding proportion of ethylene isophthalate is from 60 to 5 mol %.Preference is given to copolyesters in which the proportion of ethyleneterephthalate is from 50 to 90 mol % and the corresponding proportion ofethylene isophthalate is from 50 to 10 mol %, and very particularpreference is given to copolyesters in which the proportion of ethyleneterephthalate is from 60 to 85 mol % and the corresponding proportion ofethylene isophthalate is from 40 to 15 mol %.

[0028] Nonsealable outer layer (C):

[0029] For the other, nonsealable outer layer (C), or for any otherintermediate layers present, use in principle may be made of the samepolymers as described above for the base layer (B).

[0030] Sealing and processing properties:

[0031] The desired sealing and processing properties of the film of theinvention are obtained from the properties of the copolyester used forthe sealable outer layer combined with the topographies of the sealableouter layer (A) and the nonsealable outer layer (C).

[0032] The minimum sealing temperature of not more than 110° C. and theseal seam strength of at least 1.3 N/15 mm of film width are achievedwhen the copolymers more particularly described above are used for thesealable outer layer (A). The best sealing properties for the film areachieved when no other additives, in particular no inorganic or organicfillers, are added to the copolymer. For a given copolyester, this givesthe lowest minimum sealing temperature and the highest seal seamstrengths. However, in this case the handling of the film is poor, sincethe surface of the sealable outer layer (A) has a marked tendency toblock. The film is difficult to wind and is not at all suitable forfurther processing on high-speed packaging machinery. To improve thehandling of the film and its processability it is necessary to modifythe sealable outer layer (A). This is best done with the aid of suitableantiblocking agents of selected size, a certain amount of which is addedto the sealable layer, and specifically in such a way as firstly tominimize blocking of the film and secondly to bring about onlyinsignificant impairment of the sealing properties. Surprisingly, thisdesired combination of properties can be achieved when the topography ofthe sealable outer layer (A) is characterized by the following set ofparameters:

[0033] According to the invention, the roughness of the sealable outerlayer, expressed by the R_(a), should be less than or equal to 40 nm.Otherwise the sealing properties for the purposes of the presentinvention are adversely affected.

[0034] According to the invention, the value measured for gas flowshould be within the range from 300 to 4000 s. At values below 300 s,the sealing properties for the purposes of the present invention areadversely affected, while at values above 4000 s the handling of thefilm is impaired.

[0035] For further improvement in the processing properties of thesealable film, the topography of the nonsealable outer layer (C) shouldbe characterized by the following set of parameters:

[0036] According to the invention, the coefficient of friction (COF) ofthis side with respect to itself should be less than or equal to 0.5.Otherwise the winding performance and further processing of the film areunsatisfactory.

[0037] The roughness of the nonsealable outer layer (C), expressed asits R_(a), should be greater than or equal to 40 nm and less than orequal to 150 nm. R_(a) values below 40 nm have adverse effects on thewinding and processing performance of the film, while R_(a) values above150 nm impair the optical properties (gloss, haze) of the film.

[0038] According to the invention, the value measured for gas flow forthe outer layer (C) should be within the range below 140 s. At valuesabove 140 s, the winding and processing performance of the film isadversely affected.

[0039] The number of elevations N per mm² of film surface is correlatedwith their respective heights h via the following equation:

0.29−3.30·log h/μm<log N/mm ²<1.84−2.70·log h/μm

[0040] where

0.01 μm≦h≦10 μm

[0041] If the values for N are smaller than given by the left-hand sideof the inequality, the winding and processing performance of the film isadversely affected, and if the values for N are greater than given bythe right-hand side of the inequality, the gloss and the haze of thefilm are adversely affected.

[0042] Antiblocking agents

[0043] The base layer (B) may also comprise conventional additives, suchas stabilizers and/or antiblocking agents. The two other layers (A) and(C) may also comprise conventional additives, such as stabilizers and/orantiblocking agents. It is expedient to add the agents to the polymer orto the polymer mixture prior to melting. The stabilizers usedadvantageously comprise phosphorus compounds, for example, such asphosphoric acid or phosphoric esters.

[0044] Typical antiblocking agents (in this context also termed“pigments”) are inorganic or organic particles, such as calciumcarbonate, amorphous silica, talc, magnesium carbonate, bariumcarbonate, calcium sulfate, barium sulfate, lithium phosphate, calciumphosphate, magnesium phosphate, aluminum oxide, LiF, the calcium,barium, zinc or manganese salts of the dicarboxylic acids used, carbonblack, titanium dioxide, kaolin, or crosslinked polystyrene particles orcrosslinked acrylate particles.

[0045] The antiblocking agents selected may also be mixtures of two ormore different antiblocking agents or mixtures of antiblocking agents ofthe same composition but of different particle size. The particles maybe added to each layer in respective advantageous concentrations, e.g.as a glycolic dispersion during the polycondensation, or by way ofmasterbatches during extrusion.

[0046] Preferred particles are SiO₂ in colloidal or in chain form. Theseparticles become very well bound into the polymer matrix and create onlyvery few vacuoles. Vacuoles generally cause haze and it is thereforeappropriate to avoid these. There is no restriction in principle on thediameters of the particles used. However, it has proven appropriate forachieving the object to use particles with an average primary particlediameter below 100 nm, preferably below 60 nm and particularlypreferably below 50 nm, measured by the sedigraph method, and/orparticles with an average primary particle diameter not less than 1 μm,preferably not less than 1.5 μm and particularly preferably not lessthan 2 μm. However, the average particle diameter of these particlesdescribed last should not be above 5 μm.

[0047] To achieve the abovementioned properties of the sealable film, ithas also proven to be appropriate to select a particle concentration inthe base layer B which is lower than in the two outer layers (A) and (C). In a three-layer film of the type mentioned the particle concentrationin the base layer (B) will be from 0 to 0.15% by weight, preferably from0 to 0.12% by weight and in particular from 0 to 0.10% by weight. Thereis no restriction in principle on the diameter of the particles used,but particular preference is given to particles with an average diameternot less than 1 μm.

[0048] In its advantageous usage form, the film is composed of threelayers: the base layer (B) and, applied on both sides of this baselayer, outer layers (A) and (C), and outer layer (A) is sealable withrespect to itself and with respect to outer layer (C).

[0049] To achieve the property profile mentioned for the film, the outerlayer (C) has more pigments (i.e. a higher pigment concentration) thanthe outer layer (A). According to the invention, the pigmentconcentration in this second outer layer (C) is from 0.1 to 1.0% byweight, advantageously from 0.12 to 0.8% by weight and in particularfrom 0.15 to 0.6% by weight. In contrast, the other outer layer (A),which is sealable and positioned opposite to the outer layer (C), has alower degree of filling with inert pigments. The concentration of theinert particles in layer (A) is from 0.01 to 0.2% by weight, preferablyfrom 0.015 to 0.15% by weight and in particular from 0.02 to 0.1% byweight, all weight % ages being based on the total weight of therespective layer.

[0050] Between the base layer and the outer layers there may, ifdesired, also be an intermediate layer. This may again be composed ofthe polymers described for the base layers. In one particularlypreferred embodiment, the intermediate layer is composed of thepolyester used for the base layer. The intermediate layer may alsocomprise the customary additives described. The thickness of theintermediate layer is generally above 0.3 μm, preferably in the rangefrom 0.5 to 15 μm, in particular in the range from 1.0 to 10 μm andparticularly preferably in the range from 1.0 to 5 μm.

[0051] In the particularly advantageous three-layer embodiment of thenovel film, the thickness of the outer layers (A) and (C) is generallyabove 0.1 μm, and is generally in the range from 0.2 to 4.0 μm.particularly preferably in the range from 0.2 to 3.5 μm, in particularin the range from 0.3 to 3 μm and very particularly preferably in therange from 0.3 to 2.5 μm, and the thicknesses of the outer layers (A)and (C) may be identical or different.

[0052] The total thickness of the novel polyester film may vary withinwide limits. It is from 3 to 80 μm, in particular from 4 to 50 μm,preferably from 5 to 30 μm, the layer (B) preferably making up from 5 to90% of the total thickness.

[0053] In producing the film, the polymers for the base layer B and thetwo outer layers (A) and (C) are introduced to three extruders. Anyforeign bodies or contamination present may be removed from the polymermelt by means of suitable filters prior to extrusion. The melts are thenextruded in a coextrusion die to give flat melt films, and layered oneupon the other. The multilayer film is then drawn off and solidifiedwith the aid of a chill roll and, if desired, other rolls.

[0054] Production process:

[0055] The invention also provides a process for producing the polyesterfilm of the invention by the coextrusion process known from theliterature.

[0056] The procedure for this process is that the melts corresponding tothe individual layers (A), (B) and (C) of the film are coextrudedthrough a flat-film die, the resultant film is drawn off forsolidification on one or more rolls, the film is then biaxiallystretched (oriented), and the biaxially stretched film is heat-set and,if desired, corona- or flame-treated on the surface layer intended fortreatment.

[0057] The biaxial stretching (orientation) is generally carried outsequentially, and preference is given to sequential biaxial stretchingin which stretching is first longitudinal (in the machine direction) andthen transverse (perpendicular to the machine direction).

[0058] As is usual in coextrusion, the polymer or the polymer mixturefor the individual layers is first compressed and plasticized in anextruder, and any additives used may already be present in the polymeror the polymer mixture.

[0059] The melts are then simultaneously extruded through a flat-filmdie (slot die), and the extruded multilayer film is drawn off on one ormore take-off rolls, whereupon it cools and solidifies.

[0060] The biaxial orientation is generally carried out sequentially,preferably orienting first longitudinally (i.e. in the machinedirection=MD) and then transversely (i.e. perpendicularly to the machinedirection=TD). This gives orientation of the molecular chains. Thelongitudinal orientation can be carried out with the aid of two rollsrunning at different speeds corresponding to the desired stretchingratio. For the transverse orientation use is generally made of anappropriate tenter frame, clamping both edges of the film and thendrawing toward the two sides at an elevated temperature.

[0061] The temperature at which the orientation is carried out may varyover a relatively wide range and depends on the film properties desired.The longitudinal stretching is generally carried out at from about 80 to130° C., and the transverse stretching at from about 80 to 150° C. Thelongitudinal stretching ratio is generally in the range from 2.5:1 to6:1, preferably from 3:1 to 5.5:1. The transverse stretching ratio isgenerally in the range from 3.0:1 to 5.0:1, preferably from 3.5:1 to4.5:1. Prior to the transverse stretching, one or both surfaces of thefilm may be in-line coated by known processes. The in-line coating mayserve, for example, to give improved adhesion of a metal layer or of anyprinting ink applied, or else to improve antistatic performance orprocessing performance.

[0062] For producing a film with very good sealing properties it hasproven advantageous for the planar orientation Δp of the film to be lessthan 0.168, but particularly less than 0.165. In this case the strengthof the film in the direction of its thickness is so great that when theseal seam strength is measured it is specifically the seal seam whichseparates, and the tear does not enter the film or propagate therein.

[0063] The significant variables affecting the planar orientation Δphave been found to be the longitudinal and transverse stretchingparameters, and also the SV (standard viscosity) of the raw materialused. The processing parameters include in particular the longitudinaland transverse stretching ratios (λ_(MD) and λ_(TD)), the longitudinaland transverse stretching temperatures (T_(MD) and T_(TD)), the film webspeed and the nature of the stretching, in particular that in thelongitudinal direction of the machine. For example, if the planarorientation Δp obtained with a machine is 0.169 with the following setof parameters: λ_(MD)=4.8 and λ_(TD)=4.0, a longitudinal stretchingtemperature T_(MD) of from 80 - 118° C. and a transverse stretchingtemperature T_(TD) of from 80 - 125° C., then increasing thelongitudinal stretching temperature T_(MD) to 80 - 125° C. or increasingthe transverse stretching temperature T_(TD) to 80 - 135° C., orlowering the longitudinal stretching ratio λ_(MD) to 4.3 or lowering thetransverse stretching ratio λ_(TD) to 3.7 gives a planar orientation Δpwithin the desired range. The film web speed here was 340 m/min and theSV (standard viscosity) of the material was about 730. For thelongitudinal stretching, the data mentioned are based on what is knownas N-TEP stretching, composed of a low-orientation stretching step (LOE,Low Orientation Elongation) and a high-orientation stretching step (REP,Rapid Elongation Process). Other stretching systems in principle givethe same ratios, but the numeric values for each process parameter maybe slightly different. The temperatures given are based on therespective roll temperatures in the case of the longitudinal stretchingand on infrared-measured film temperatures in the case of the transversestretching.

[0064] In the heat-setting which follows, the film is held for from 0.1to 10 s at a temperature of from 150 to 250° C. The film is then woundup in a usual manner.

[0065] After the biaxial stretching it is preferable for one or bothsurfaces of the film to be corona- or flame-treated by one of the knownmethods. The intensity of the treatment is generally in the range above45 mN/m.

[0066] The film may also be coated in order to achieve other desiredproperties. Typical coatings are layers with adhesion-promoting,antistatic, slip-improving or release action. These additional layersmay, it will be appreciated, be applied to the film by way of in-linecoating, using aqueous dispersions, prior to the transverse stretchingstep.

[0067] Advantages of the invention:

[0068] The novel film has excellent sealability, very good handlingproperties and very good processing performance. The sealable outerlayer (A) of the film seals not only with respect to itself (finsealing) but also with respect to the nonsealable outer layer (C) (lapsealing). The minimum sealing temperature for the lap sealing here isonly about 10 K higher, and the reduction in the seal seam strength isnot more than 0.3 N/15 mm.

[0069] The gloss and haze of the film are also improved considerably. Inproducing the novel film it is certain that material for recycling canbe refed to the extrusion process at a concentration of from 20 to 60%by weight, based on the total weight of the film, without anysignificant adverse effect on the physical properties of the film, inparticular on its appearance.

[0070] The film is accordingly very useful in flexible packaging,especially where its excellent sealing properties and its goodprocessability come in useful. This is in particular its use onhigh-speed packaging machines.

[0071] The most important film properties according to the invention canbe seen again at a glance in the table below (Table 1). TABLE 1 Rangeaccording Particu- to the Pre- larly Test invention ferred preferredUnit method OUTER LAYER A Minimum <110 <105 <100 ° C. internal sealingtemperature Seal seam >1.3 >1.5 >1.8 N/15 mm internal strength Average≦40 ≦30 ≦20 nm DIN 4768, roughness R_(a) cut-off of 0.25 mm Range of300-4000 500-3500 1000-3000 sec internal values measured for gas flowGloss, 20° >120 >130 >140 DIN 67530 OUTER LAYER C COF <0.5 <0.45 <0.40DIN 53375 Average 40 to 150 45 to 120 50 to 90 nm DIN 4768, roughnessR_(a) cut-off of 0.25 mm Range of ≦140 ≦120 ≦100 sec internal valuesmeasured for gas flow Constants 0.29/3.00 A₁/A₂ and B₁/B₂ and 1.84/2.7Gloss, 20° C. >140 >150 >160 DIN 67530 Other film properties Haze <4 <3<2.5 % ASTM-D 1003-52 Planar <0.168 <0.165 <0.163 internal orientation

[0072] The following test methods were utilized for the purposes of thepresent invention to characterize the raw materials and the films:

[0073] SV (standard viscosity)

[0074] The standard viscosity SV (DCA) is measured in dichloroaceticacid by a method based on DIN 53726.

[0075] The intrinsic viscosity (IV) is calculated as follows from thestandard viscosity

IV (DCA)=6.907·10⁻⁴ SV (DCA)+0.063096

[0076] Determination of minimum sealing temperature

[0077] Hot-sealed specimens (seal seam 20 mm×100 mm) are produced with aBrugger HSG/ET sealing apparatus, by sealing the film at differenttemperatures with the aid of two heated sealing jaws at a sealingpressure of 2 bar and with a sealing time of 0.5 s. From the sealedspecimens test strips of 15 mm width were cut. The T-seal seam strengthwas measured as in the determination of seal seam strength. The minimumsealing temperature is the temperature at which a seal seam strength ofat least 0.5 N/15 mm is achieved.

[0078] Seal seam strength

[0079] To determine the seal seam strength, two film strips of width 15mm were placed one on top of the other and sealed at 130° C. with asealing time of 0.5 s and a sealing pressure of 2 bar (apparatus:Brugger model NDS, single-side-heated sealing jaw). The seal seamstrength was determined by the T-peel method.

[0080] Coefficient of friction

[0081] The coefficient of friction was determined to DIN 53 375. Thecoefficient of sliding friction was measured 14 days after production.

[0082] Surface tension

[0083] Surface tension was determined by what is known as the ink method(DIN 53 364).

[0084] Haze

[0085] The Hölz haze was measured by a method based on ASTM-D 1003-52but, in order to utilize the most effective measurement range,measurements were made on four pieces of film laid one on top of theother, and a 1° slit diaphragm was used instead of a 4° pinhole.

[0086] Gloss

[0087] Gloss was determined to DIN 67 530. The reflectance was measuredas an optical value characteristic of a film surface. Based on thestandards ASTM-D 523-78 and ISO 2813, the angle of incidence was set at20° or 60°. A beam of light hits the flat test surface at the set angleof incidence and is reflected and/or scattered thereby. A proportionalelectrical variable is displayed representing light rays hitting thephotoelectronic detector. The value measured is dimensionless and mustbe stated together with the angle of incidence.

[0088] Determination of particle sizes on film surfaces

[0089] A scanning electron microscope and an image analysis system areused to determine the size distribution of elevations on film surfaces.Use is made of the XL30 CP scanning electron microscope from Philipswith an integrated image analysis program: AnalySIS from Soft-ImagingSystem.

[0090] For these measurements, specimens of film are placed flat on aspecimen holder. These are then metalized obliquely at an angle a with athin metallic layer (e.g. of silver) . The symbol a here is the anglebetween the surface of the specimen and the direction of diffusion ofthe metal vapor. This oblique metalization throws a shadow behind theelevation. Since the shadows are not at this stage electricallyconductive, the specimen is then further sputtered or metalized with asecond metal (e.g. gold), the second coating here impacting verticallyonto the surface of the specimen in such a way as not to produce anyshadows in the second coating.

[0091] Scanning electron microscope (SEM) images are taken of thespecimen surfaces prepared in this way. The shadows of the elevationsare visible because of the contrast of the metallic materials. Thespecimen is oriented in the SEM in such as way that the shadows runparallel to one edge of the image. The following conditions are set inthe SEM for recording the image: secondary electron detector, operatingdistance 10 mm, acceleration voltage 10 kV and spot 4.5. The brightnessand contrast are set in such a way that all of the information in theimage is represented as gray values and the intensity of the backgroundnoise is sufficiently small for it not to be detected as a shadow. Thelength of the shadows is measured by image analysis. The threshold valuefor shadow identification is set at the point where the secondderivative of the gray value distribution of the image passes throughthe zero point. Before shadow identification, the image is smoothed withan N×N filter (size 3, 1 iteration). A frame is set so as to ensure thatelevations which are not reproduced in their entirety in the image arenot included in the measurements. The magnification, the size of theframe and the number of images evaluated are selected in such a way thata total film surface of 0.36 mm² is evaluated.

[0092] The height of the individual elevations is computed from theindividual shadow lengths using the following relationship:

h=(tan α) * L

[0093] where h is the height of the elevation, α is the metalizationangle and L is the shadow length. The elevations recorded in this wayare classified so as to arrive at a frequency distribution. Theclassification is into classes of 0.05 mm width between 0 and 1 mm, thesmallest class (from 0 to 0.05 mm) not being used for further evaluationcalculations. The diameters (dimension perpendicular to the direction ofshadow throw) of the elevations are classified in a similar way inclasses of 0.2 mm width from 0 to 10 mm, and here again the smallestclass is again used for further evaluation.

[0094] Surface gas flow time

[0095] The principle of the test method is based on the air flow betweenone side of the film and a smooth silicon wafer sheet. The air flowsfrom the surroundings into an evacuated space, and the interface betweenfilm and silicon wafer sheet acts as a flow resistance.

[0096] A round specimen of film is placed on a silicon wafer sheet inthe middle of which there is a hole providing the connection to thereceiver. The receiver is evacuated to a pressure below 0.1 mbar. Thetime in seconds taken by the air to establish a pressure rise of 56 mbarin the receiver is determined.

[0097] Test conditions: Test area 45.1 cm² Weight applied 1276 g Airtemperature 23° C. Humidity 50% relative humidity Aggregated gas volume1.2 cm³ Pressure difference 56 mbar

[0098] Determination of planar orientation Δp

[0099] Planar orientation is determined by measuring the refractiveindex with an Abbe refractometer according to internal operatingprescription 24.

[0100] Preparation of specimens

[0101] Specimen size and length: from 60 to 100 mm

[0102] Specimen width: corresponds to prism width of 10 mm

[0103] To determine n_(MD) and n_(α) (=n_(z)), the specimen to be testedhas to be cut out from the film with the running edge of the specimenrunning precisely in the direction TD. To determine n_(TD) and n_(α)(=n_(z)), the specimen to be tested has to be cut out from the film withthe running edge of the specimen running precisely in the direction MD.The specimens are to be taken from the middle of the film web. Care mustbe taken that the temperature of the Abbe refractometer is 23° C. Usinga glass rod, a little diiodomethane (N=1.745) ordiiodomethane-bromo-naphthalene mixture is applied to the lower prism,which has been cleaned thoroughly before the test. The refractive indexof the mixture must be greater than 1.685. The specimen cut out in thedirection TD is firstly laid on top of this, in such a way that theentire surface of the prism is covered. Using a paper wipe the film isnow firmly pressed flat onto the prism, so that it is firmly andsmoothly positioned thereon. The excess liquid must be sucked away. Alittle of the test liquid is then dropped onto the film. The secondprism is swung down and into place and pressed firmly into contact. Theright-hand knurled screw is then used to turn the indicator scale untila transition from light to dark can be seen in the field of view in therange from 1.62 to 1.68. If the transition from light to dark is notsharp, the colors are brought together using the upper knurled screw insuch a way that only one light and one dark zone are visible. The sharptransition line is brought to the crossing point of the two diagonallines (in the eyepiece) using the lower knurled screw. The value nowindicated on the measurement scale is read off and entered into the testrecord. This is the refractive index n_(MD) in the machine direction.The scale is now turned using the lower knurled screw until the rangevisible in the eyepiece is from 1.49 to 1.50.

[0104] The refractive index n_(α) or n_(z) (in the direction of thethickness of the film) is then determined. To improve the visibility ofthe transition, which is only weakly visible, a polarization film isplaced over the eyepiece. This is turned until the transition is clearlyvisible. The same considerations apply as in the determination ofn_(MD). If the transition from light to dark is not sharp (colored), thecolors are brought together using the upper knurled screw in such a waythat a sharp transition can be seen. This sharp transition line isbrought into the crossing point of the two diagonal lines using thelower knurled screw, and the value indicated on the scale is read offand entered into the table.

[0105] The specimen is then turned, and the corresponding refractiveindices n_(MD) and n_(α) (=n_(z)) of the other side are measured andentered into an appropriate table.

[0106] After determining the refractive indices in, respectively, thedirection MD and the direction of the thickness of the film, thespecimen strip cut out in the direction MD is placed in position and therefractive indices n_(TD) and n_(α) (=n_(z)) are determined accordingly.The strip is turned over, and the values for the B side are measured.The values for the A side and the B side are combined to give averagerefractive indices. The orientation values are then calculated from therefractive indices using the following formulae:

Δn=n _(MD) −n _(TD)

Δp=(n _(MD) +n _(TD))/2−n _(z)

n _(av)=(n _(MD) +n _(TD) +n _(z))/3

EXAMPLE 1

[0107] Chips made from polyethylene terephthalate (prepared by thetransesterification process with Mn as transesterification catalyst, Mnconcentration: 100 ppm) were dried at 150° C. to residual moisture below100 ppm and fed to the extruder for the base layer (B) . Chips made frompolyethylene terephthalate and a filler were likewise fed to theextruder for the nonsealable outer layer (C).

[0108] Alongside this, chips were prepared made from a linear polyesterwhich is composed of an amorphous copolyester with 78 mol % of ethyleneterephthalate and 22 mol % of ethylene isophthalate (prepared via thetransesterification process with Mn as transesterification catalyst, Mnconcentration: 100 ppm). The copolyester was dried at a temperature of100° C. to a residual moisture below 200 ppm and fed to the extruder forthe sealable outer layer (A).

[0109] Coextrusion followed by stepwise longitudinal and transverseorientation was then used to produce a transparent, three-layer filmwith ABC structure and with a total thickness of 12 μm. The thickness ofeach outer layer can be seen in Table 2. Outer layer (A), a mixture madefrom: 97.0% by weight of copolyester with an SV of 800 3.0% by weight ofmasterbatch made from 97.75% by weight of copolyester (SV of 800) and1.0% by weight of ® Sylobloc 44 H (synthetic SiO₂ from Grace) and 1.25%by weight of ® Aerosil TT 600 (pyrogenic SiO₂ from Degussa) Base layer(B): 100.0% by weight of polyethylene terephthalate with an SV of 800Outer layer (C), a mixture made from: 88% by weight of polyethyleneterephthalate with an SV of 800 12% by weight of masterbatch made from97.75% by weight of copolyester (SV of 800) and 1.0% by weight ofSylobloc 44 H (synthetic SiO₂ from Grace) and 1.25% by weight of AerosilTT 600 (chain- type SiO₂ from Degussa) The production conditions in eachprocess step were: Extrusion: Temperatures Layer A: 3e+08 ° C. Layer B:° C. Layer C: ° C. Die width: 2.5 mm Take-off roll 30° C. temperatureLongitudinal Temperature: 80-125° C. stretching: Longitudinal stretching4.2 ratio: Transverse Temperature: 80-135° C. stretching: Transversestretching 4 ratio Heat-setting: Temperature: 230° C. Duration: 3 s

[0110] The film had the required good sealing properties and therequired handling, and the required processing performance. Tables 2 and3 show the structure of the films and the properties achieved in filmsproduced in this way.

EXAMPLE 2

[0111] As Example 1, except that the outer layer thickness for thesealable layer (A) was raised from 1.5 to 2.0 μm, with otherwiseidentical film structure and an identical method of production. Thisgives an improvement in the sealing properties, and in particular theseal seam strength improves markedly.

EXAMPLE 3

[0112] As Example 1, except that a film of 20 μm thickness was produced.The outer layer thickness for the sealable layer (A) was 2.5 μm and thethickness for the nonsealable layer (C) was 2.0 μm. This again improvesthe sealing properties, and in particular the seal seam strengthimproves markedly. Again, there has been a slight improvement in thehandling of the film.

EXAMPLE 4

[0113] As Example 3, except that the copolymer for the sealable outerlayer (A) was changed. Instead of the amorphous copolyester having 78mol % of polyethylene terephthalate and 22 mol % of ethyleneisophthalate, use was made of an amorphous copolyester having 70 mol %of polyethylene terephthalate and 30 mol % of ethylene isophthalate. Thepolymer was processed on a vented twin-screw extruder but did not haveto be predried. The outer layer thickness for the sealable layer (A) wasagain 2.5 μm, and the thickness of the nonsealable layer (C) was 2.0 μm.This improved the sealing properties, and in particular there was amarked improvement in the seal seam strength. To achieve good handlingand good processing performance from the film, the pigment concentrationin the two outer layers was slightly raised.

COMPARATIVE EXAMPLE 1

[0114] As Example 1, except that the sealable outer layer (A) wasunpigmented. Although this has improved the sealing properties somewhat,the handling of the film and its processing performance havedeteriorated unacceptably.

COMPARATIVE EXAMPLE 2

[0115] As Example 1, except that the sealable outer layer (A) had thesame pigmentation level as the nonsealable outer layer (C). This measurehas improved the handling and the processing properties of the film,however the sealing properties have become markedly poorer.

COMPARATIVE EXAMPLE 3

[0116] As Example 1, except that the nonsealable outer layer (A) wasgiven markedly less pigmentation. The handling and processingperformance of the film has become markedly poorer.

COMPARATIVE EXAMPLE 4

[0117] Example 1 from EP-A-0 035 835 was repeated. The sealingperformance of the film, its handling properties and its processingperformance are poorer than in the examples according to the invention.TABLE 2 Film Layer thicknesses Average pigment diameter Pigmentconcentrations thickness Film μm Pigments in layers in layers μm ppmExample μm structure A B C A B C A B C A B C E 1 12 ABC 1.5 9 1.5Sylobloc 44 H none Sylobloc 44 H 2.5 2.5 300 0 1200 Aerosil TT 600Aerosil TT 600 0.04 0.04 375 1500 E 2 12 ABC 2 8.5 1.5 Sylobloc 44 Hnone Sylobloc 44 H 2.5 2.5 300 0 1200 Aerosil TT 600 Aerosil TT 600 0.040.04 375 1500 E 3 20 ABC 2.5 15.5 2 Sylobloc 44 H none Sylobloc 44 H 2.52.5 300 0 1200 Aerosil TT 600 Aerosil TT 600 0.04 0.04 375 1500 E 4 20ABC 2.5 15.5 2 Sylobloc 44 H none Sylobloc 44 H 2.5 2.5 400 0 1500Aerosil TT 60 Aerosil TT 600 0.04 0.04 500 1875 CE 1 12 ABC 1.5 9 1.5none none Sylobloc 44 H 2.5 0 1200 Aerosil TT 600 0.04 1500 CE 2 12 ABC1.5 9 1.5 Sylobloc 44 H none Sylobloc 44 H 2.5 2.5 300 0 1200 Aerosil TT600 Aerosil TT 600 0.04 0.04 375 1500 CE 3 12 ABC 1.5 9 1.5 Sylobloc 44H none Sylobloc 44 H 2.5 2.5 300 0  600 Aerosil TT 600 Aerosil TT 6000.04 0.04 375  750 CE 4 15 AB 2.25 12.8 Gasil 35 none 3 2500  0

[0118] TABLE 3 Minimum Coeffi- sealing cient of temperature Seal seamfriction Average Winding ° C. strength COF roughness Values perform- Aside A side C side R_(a) measured for ance and Process- Ex- with withwith nm gas flow Constants Gloss handling ing am- respect respectrespect A C A C A/B A C proper- perform- ple to A side to A side to Cside side side side side A C Δp side side Haze ties ance E 1 100 2.00.45 25 65 1200 120 0.5 3.1 0.17 140 170 2.5 ++ ++ E 2  98 2.7 0.45 2665 1280 120 0.5 3.1 0.17 140 170 2.5 ++ ++ E 3  95 3 0.41 23 61 1110 1200.5 3.1 0.17 130 170 3 ++ ++ E 4  85 3.3 0.4  23 65 1300 110 0.5 3.10.17 130 170 3 ++ ++ CE 1  98 2.1 0.45 10 65 10,000  80 0.17 160 170 1.5− − CE 2 110 1 0.45 65 65  80  80 0.17 130 170 2.8 − − CE 3 100 2 0.4525 37 1200 150 0.17 160 190 1.5 − − CE 4 115 0.97 >2 70 20  50 >5000 12− −

What is claimed is:
 1. A coextruded, biaxially oriented, sealablepolyester film with at least a base layer (B), with a sealable outerlayer (A) arranged on outside of the base layer (B) and with anotherouter layer (C) arranged on the other side of the base layer (B), wherethe sealable outer layer (A) has a minimum sealing temperature of notmore than about 110° C. and a seal seam strength of at least about 1.3N/15 mm of film width, wherein the sealable outer layer (A) has anaverage surface roughness, expressed as R_(a), of ≦40 nm, and has avalue measured for gas flow within the range from about 300 to about4000 s, wherein the nonsealable outer layer (C) has a coefficient offriction COF of≦0.5, an average surface roughness, expressed as R_(a),within the range of 40≦R_(a)≦150 nm, and a value measured for gas flowof ≦140 s, and wherein, for the nonsealable outer layer (C), the numberof elevations N_(c) per mm² of film surface is correlated with theirrespective heights h via the following equations: A _(c1) −B _(c1)·logh/μm<log N _(c) /mm ² <A _(c2) −B _(c2)·log h/μm where 0.01 μm≦h≦10 μmand A _(c1)=0.29, B _(c1)=3.30 and A _(c2)=1.84, B _(c2)=2.70.
 2. Thesealable polyester film as claimed in claim 1 , wherein the sealableouter layer (A) comprises an amorphous copolyester composed of ethyleneterephthalate units and of ethylene isophthalate units and of ethyleneglycol units.
 3. The sealable polyester film as claimed in claim 1 ,wherein the amorphous copolyester of the sealable outer layer (A)comprises from about 40 to about 95 mol % of ethylene terephthalate andfrom about 60 to about 5 mol % of ethylene isophthalate.
 4. The sealablepolyester film as claimed in claim 1 , wherein the sealable outer layer(A) has a thickness in the range from about 0.2 to about 3 μm.
 5. Thesealable polyester film as claimed in claim 1 , which comprisesantiblocking agents selected from the group consisting of inorganic andorganic particles and mixtures of these.
 6. The sealable polyester filmas claimed in claim 5 , which comprises, as antiblocking agent,particles with an average primary particle diameter below about 100 nm,measured by the sedigraph method, or particles with an average primaryparticle diameter greater than or equal to about 1 W, or particles withan average primary particle diameter below about 100 nm measured by thesedigraph method and particles with an average primary particlediameter≧about 1 μm.
 7. The sealable polyester film as claimed in claim6 , wherein the amount of particles in the base layer (B) is within therange from about 0 to about 0.15% by weight, wherein the amount ofparticles in the nonsealable outer layer (C) is within the range fromabout 0.1 to about 1.0% by weight, and wherein the sealable outer layer(A) has an amount of particles within the range from about 0.01 to about0.2% by weight, all data being based on % by weight and on the totalweight of the respective layer.
 8. The sealable polyester film asclaimed in claim 1 , wherein the thicknesses of the outer layers (A) and(C) are above about 0.1 μm the thicknesses of the outer layers (A) and(C) being identical or different.
 9. The sealable polyester film asclaimed in claim 1 , wherein the total thickness of the polyester filmis within the range from about 3 to about 80 μm, the base layer (B)making up a proportion of from about 5 to about 90% of the totalthickness.
 10. A process for producing a sealable polyester film asclaimed in claim 1 , in which the polymers for the base layer (B) andthe two outer layers (A) and (C) are fed to separate extruders, themelts are then coextruded through a coextrusion die to give a flat meltfilm and then the film is drawn off with the aid of a chill roll andsolidified, and then biaxially stretch-oriented and heat-set, and then,where appropriate, corona- or flame-treated on at least one surface,which process comprises carrying out the biaxial stretching insuccession, first stretching longitudinally (in the machine direction)and then transversely (perpendicularly to the machine direction),carrying out the longitudinal stretching at a temperature within therange from about 80 to about 130° C., and the transverse stretchingwithin the range from about 90 to about 150° C., and setting thelongitudinal stretching ratio within the range from about 2.5:1 to about6:1, and the transverse stretching ratio within the range from about3.0:1 to about 5.0:1.
 11. The process as claimed in claim 10 , whereinone or both surfaces of the film are in-line coated after thelongitudinal stretching and prior to the transverse stretching.